Here's a breakdown of how to use it:
* Formal: "Merci beaucoup" (Thank you very much)
* Informal: "Merci" (Thank you)
Keep in mind that Belgium is a bilingual country, so you might also hear "Bedankt" which is the Dutch translation of "thank you".
However, "Merci" is widely understood and used throughout the country, regardless of the language spoken.